Dynamic measurements of total hepatic blood flow with Phase Contrast MRI

In Vitro to In Vivo Scalars for Drug Clearance in Nonalcoholic Fatty Liver and Steatohepatitis

In vitro-in vivo extrapolation (IVIVE) allows prediction of clinical outcomes across populations from in vitro data using specific scalars tailored to the biologic characteristics of each population. This study experimentally determined scalars for patients with varying degrees of nonalcoholic fatty liver disease (NAFLD), ranging from fatty liver to nonalcoholic steatohepatitis (NASH) and cirrhosis. Microsomal, S9, and cytosol fractions were extracted from 36 histologically normal and 66 NAFLD livers (27 nonalcoholic fatty liver [NAFL], 13 NASH, and 26 NASH with cirrhosis). Corrected microsomal protein per gram liver (MPPGL) progressively decreased with disease severity (26.8, 27.4, and 24.3 mg/g in NAFL, NASH, and NASH/cirrhosis, respectively, compared with 35.6 mg/g in normal livers; ANOVA, P < 0.001). Homogenate, S9, and cytosolic protein showed a consistent trend of decline in NASH/cirrhosis relative to normal control (post-hoc t test, P < 0.05). No differences across the groups were observed in homogenate, S9, cytosolic, and microsomal protein content in matched kidney samples. MPPGL-based scalars that combine protein content with liver size revealed that the reduction in MPPGL in NAFL and NASH was compensated by the reported increase in liver size (relative scalar ratios of 0.96 and 0.99, respectively), which was not the case with NASH/cirrhosis (ratio of 0.63), compared with healthy control. Physiologically based pharmacokinetics-informed global sensitivity analysis of the relative contribution of IVIVE scalars (hepatic CYP3A4 abundance, MPPGL, and liver size) to variability in exposure (area under the curve) to three CYP3A substrates (alprazolam, midazolam, and ibrutinib) revealed enzyme abundance as the most significant parameter, followed by MPPGL, whereas liver volume was the least impactful factor. SIGNIFICANCE STATEMENT: Nonalcoholic fatty liver disease-specific scalars necessary for extrapolation from in vitro systems to liver tissue are lacking. These are required in clearance prediction and dose selection in nonalcoholic fatty liver and steatohepatitis populations. Previously reported disease-driven changes have focused on cirrhosis, with no data on the initial stages of liver disease. The authors obtained experimental values for microsomal, cytosolic, and S9 fractions and assessed the relative impact of microsomal scalars on predicted exposure to substrate drugs using physiologically based pharmacokinetics.

4D Flow MRI in the portal venous system: imaging and analysis methods, and clinical applications

Thus far, ultrasound, CT, and 2D cine phase-contrast MRI has been adopted to evaluate blood flow and vascular morphology in the portal venous system; however, all these techniques have some shortcomings, such as limited field of view and difficulty in accurately evaluating blood flow. A new imaging technique, namely 3D cine phase-contrast (4D Flow) MRI, can acquire blood flow data of the entire abdomen at once and in a time-resolved manner, allowing visual, quantitative, and comprehensive assessment of blood flow in the portal venous system. In addition, a retrospective blood flow analysis, i.e., "retrospective flowmetry," is possible. Although the development of 4D Flow MRI for the portal system has been delayed compared to that for the arterial system owing to the lower flow velocity of the portal venous system and the presence of respiratory artifacts, several useful reports have recently been published as the technology has advanced. In the first part of this narrative review article, technical considerations of image acquisition and analysis methods of 4D Flow MRI for the portal venous system and the validations of their results are described. In the second part, the current clinical application of 4D Flow MRI for the portal venous system is reviewed.

Multiscale biomechanics and mechanotransduction from liver fibrosis to cancer

A growing body of multiscale biomechanical studies has been proposed to highlight the mechanical cues in the development of hepatic fibrosis and cancer. At the cellular level, changes in mechanical microenvironment induce phenotypic and functional alterations of hepatic cells, initiating a positive feedback loop that promotes liver fibrogenesis and hepatocarcinogenesis. Tumor mechanical microenvironment of hepatocellular carcinoma facilitates tumor cell growth and metastasis, and hinders the drug delivery and immunotherapy. At the molecular level, mechanical forces are sensed and transmitted into hepatic cells via allosteric activation of mechanoreceptors on the cell membrane, leading to the activation of various mechanotransduction pathways including integrin and YAP signaling and then regulating cell function. Thus, the application of mechanomedicine concept in the treatment of liver diseases is promising for rational design and cell-specific delivery of therapeutic drugs. This review mainly discusses the correlation between biomechanical cues and liver diseases from the viewpoint of mechanobiology.

Portal Vein Pulsatility Index as a Potential Risk of Venous Congestion Assessed by Magnetic Resonance Imaging: A Prospective Study on Healthy Volunteers

High values of the portal vein pulsatility index (PI) have been associated with adverse outcomes in perioperative or critically ill patients. However, data on dynamic changes of PI related to fluid infusion are scarce. We aimed to determine if dynamic changes in PI are associated with the fluid challenge (FC). To address this challenge, we conducted a prospective single-center study. The population study included healthy subjects. FC consisted in the administration of 500 ml of Ringer lactate infusion over 5 min. The portal blood flow and PI were assessed by magnetic resonance imaging. The responsiveness to FC was defined as an increase in the cardiac stroke volume of at least 10% as assessed by echocardiography. We included 24 healthy volunteers. A total of fourteen (58%) subjects were responders, and 10 (42%) were non-responders. In the responder group, FC induced a significant increase in portal blood flow from 881 (762–1,001) at the baseline to 1,010 (778–1,106) ml min⁻¹ (p = 0.005), whilst PI remained stable (from 31 [25–41] to 35 (25–42) %; p = 0.12). In the non-responder group, portal blood flow remained stable after FC (from 1,042 to 1,034 ml min⁻¹; p = 0.084), whereas PI significantly increased from 32 (22–40) to 48% *(25–85) after FC (p = 0.027). PI was negatively correlated to portal blood flow (Rho coefficient = −0.611; p = 0.002). To conclude, PI might be a sensitive marker of early congestion in healthy subjects that did not respond to FC. This finding requires further validation in clinical settings with a larger sample size.

Clinical Applications of 4D Flow MRI in the Portal Venous System

Evaluation of the hemodynamics in the portal venous system plays an essential role in many hepatic pathologies. Changes in portal flow and vessel morphology are often indicative of disease.Routinely used imaging modalities, such as CT, ultrasound, invasive angiography, and MRI, often focus on either hemodynamics or anatomical imaging. In contrast, 4D flow MRI facilitiates a more comprehensive understanding of pathophysiological mechanisms by simultaneously and noninvasively acquiring time-resolved flow and anatomical information in a 3D imaging volume.Though promising, 4D flow MRI in the portal venous system is especially challenging due to small vessel calibers, slow flow velocities, and breathing motion. In this review article, we will discuss how to account for these challenges when planning and conducting 4D flow MRI acquisitions in the upper abdomen. We will address patient preparation, sequence acquisition, postprocessing, quality control, and analysis of 4D flow data.In the second part of this article, we will review potential clinical applications of 4D flow MRI in the portal venous system. The most promising area for clinical utilization is the diagnosis and grading of liver cirrhosis and its complications. Relevant parameters acquired by 4D flow MRI include the detection of reduced or reversed flow in the portal venous system, characterization of portosystemic collaterals, and impaired response to a meal challenge. In patients with cirrhosis, 4D flow MRI has the potential to address the major unmet need of noninvasive detection of gastroesophageal varices at high risk for bleeding. This could replace many unnecessary, purely diagnostic, and invasive esophagogastroduodenoscopy procedures, thereby improving patient compliance with follow-up. Moreover, 4D flow MRI offers unique insights and added value for surgical planning and follow-up of multiple hepatic interventions, including transjugular intrahepatic portosystemic shunts, liver transplantation, and hepatic disease in children. Lastly, we will discuss the path to clinical implementation and remaining challenges.

Hepatectomy-Induced Alterations in Hepatic Perfusion and Function - Toward Multi-Scale Computational Modeling for a Better Prediction of Post-hepatectomy Liver Function

Liver resection causes marked perfusion alterations in the liver remnant both on the organ scale (vascular anatomy) and on the microscale (sinusoidal blood flow on tissue level). These changes in perfusion affect hepatic functions via direct alterations in blood supply and drainage, followed by indirect changes of biomechanical tissue properties and cellular function. Changes in blood flow impose compression, tension and shear forces on the liver tissue. These forces are perceived by mechanosensors on parenchymal and non-parenchymal cells of the liver and regulate cell-cell and cell-matrix interactions as well as cellular signaling and metabolism. These interactions are key players in tissue growth and remodeling, a prerequisite to restore tissue function after PHx. Their dysregulation is associated with metabolic impairment of the liver eventually leading to liver failure, a serious post-hepatectomy complication with high morbidity and mortality. Though certain links are known, the overall functional change after liver surgery is not understood due to complex feedback loops, non-linearities, spatial heterogeneities and different time-scales of events. Computational modeling is a unique approach to gain a better understanding of complex biomedical systems. This approach allows (i) integration of heterogeneous data and knowledge on multiple scales into a consistent view of how perfusion is related to hepatic function; (ii) testing and generating hypotheses based on predictive models, which must be validated experimentally and clinically. In the long term, computational modeling will (iii) support surgical planning by predicting surgery-induced perfusion perturbations and their functional (metabolic) consequences; and thereby (iv) allow minimizing surgical risks for the individual patient. Here, we review the alterations of hepatic perfusion, biomechanical properties and function associated with hepatectomy. Specifically, we provide an overview over the clinical problem, preoperative diagnostics, functional imaging approaches, experimental approaches in animal models, mechanoperception in the liver and impact on cellular metabolism, omics approaches with a focus on transcriptomics, data integration and uncertainty analysis, and computational modeling on multiple scales. Finally, we provide a perspective on how multi-scale computational models, which couple perfusion changes to hepatic function, could become part of clinical workflows to predict and optimize patient outcome after complex liver surgery.

Effect of ingesting a meal and orthostasis on the regulation of splanchnic and systemic hemodynamics and the responsiveness of cardiovascular α1-adrenoceptors

Postprandial orthostasis activates mechanisms of cardiovascular homeostasis to maintain normal blood pressure (BP) and adequate blood flow to vital organs. The underlying mechanisms of cardiovascular homeostasis in postprandial orthostasis still require elucidation. Fourteen healthy volunteers were recruited to investigate the effect of an orthostatic challenge (60°-head-up-tilt for 20 min) on splanchnic and systemic hemodynamics before and after ingesting an 800-kcal composite meal. The splanchnic circulation was assessed by ultrasonography of the superior mesenteric and hepatic arteries and portal vein. Systemic hemodynamics were assessed noninvasively by continuous monitoring of BP, heart rate (HR), cardiac output (CO), and the pressor response to an intravenous infusion on increasing doses of phenylephrine, an α₁-adrenoceptor agonist. Neurohumoral regulation was assessed by spectral analysis of HR and BP, plasma catecholamine and aldosterone levels and plasma renin activity. Postprandial mesenteric hyperemia was associated with an increase in CO, a decrease in SVR and cardiac vagal tone, and reduction in baroreflex sensitivity with no change in sympathetic tone. Arterial α₁-adrenoceptor responsiveness was preserved and reduced in hepatic sinusoids. Postprandial orthostasis was associated with a shift of 500 mL of blood from mesenteric to systemic circulation with preserved sympathetic-mediated vasoconstriction. Meal ingestion provokes cardiovascular hyperdynamism, cardiac vagolysis, and resetting of the baroreflex without activation of the sympathetic nervous system. Meal ingestion also alters α₁-adrenoceptor responsiveness in the hepatic sinusoids and participates in the redistribution of blood volume from the mesenteric to the systemic circulation to maintain a normal BP during orthostasis.NEW & NOTEWORTHY A unique integrated investigation on the effect of meal on neurohumoral mechanisms and blood flow redistribution of the mesenteric circulation during orthostasis was investigated. Food ingestion results in cardiovascular hyperdynamism, reduction in cardiac vagal tone, and baroreflex sensitivity and causes a decrease in α₁-adrenoceptor responsiveness only in the venous intrahepatic sinusoids. About 500-mL blood shifts from the mesenteric to the systemic circulation during orthostasis. Accordingly, the orthostatic homeostatic mechanisms are better understood.

DNA lesions correlate with lymphocyte function after selective internal radiotherapy

In patients with non-resectable hepatic malignancies selective internal radiotherapy (SIRT) with yttrium-90 is an effective therapy. However, previous data indicate that SIRT leads to impaired immune function. The aim of the current study was to determine the extent of DNA lesions in peripheral blood mononuclear cells of SIRT patients and to correlate these lesions with cellular immune responses. In ten patients γH2AX and 53BP1 foci were determined. These foci are markers of DNA double-strand breaks (DSBs) and occur consecutively. In parallel, lymphocyte proliferation was assessed after stimulation with the T cell mitogen phytohemagglutinin. Analyses of vital cells were performed prior to and 1 h and 1 week after SIRT. 1 h and 1 week after SIRT numbers of γH2AX and of 53BP1 foci were more than threefold larger than before (p < 0.01). Already at baseline, foci were more abundant than published in healthy controls. Lymphocyte proliferation at baseline was below the normal range and further decreased after SIRT. Prior to therapy, there was an inverse correlation between lymphocyte proliferation and the quotient 53BP1/γH2AX; which could be considered as a measure of the course of DNA DSB repair (r = - 0.94, p < 0.0001). Proliferative responses were inversely correlated with 53BP1 foci prior to therapy and γH2AX and 53BP1 foci 1 h after therapy (r < - 0.65, p < 0.05). In conclusion, DNA foci in SIRT patients were correlated with impaired in vitro immune function. Unrepaired DNA DSBs or cell cycle arrest due to repair may cause this impairment.

Effect of gravity on portal venous flow: Evaluation using multiposture MRI

BACKGROUND

Analysis of portal venous flow (PVF) is important when evaluating the severity and prognosis of liver disease. PVF might be altered by postural changes (ie, difference in the effects of gravity).

PURPOSE

To evaluate the effect of gravity on PVF using a novel MRI system, which can obtain abdominal MRIs in both the supine and the upright positions.

STUDY TYPE

Prospective self control.

SUBJECTS

Twelve healthy young male volunteers.

FIELD STRENGTH/SEQUENCE

Caval velocity-mapped images were obtained using the electrocardiography-triggered cine phase-contrast technique in the supine and upright positions with multiposture MRI (paired 0.4 T permanent magnets).

ASSESSMENT

The mean PVF velocity in the region of interest in each cardiac phase was determined. A PVF curve in the cardiac cycle was also obtained from the PVF velocity multiplied by the cross-sectional area. The mean PVF velocity, maximum PVF velocity, cross-sectional area of the PV, mean PVF, maximum PVF, and heart rate in the supine and upright positions were assessed.

STATISTICAL TESTS

Wilcoxon signed-rank tests were applied. P < 0.05 was considered statistically significant.

RESULTS

The mean PVF velocity, maximum PVF velocity, cross-sectional area of the PV, mean PVF, and maximum PVF were all significantly lower in the upright position compared with the supine position (P = 0.002 for all), with differences of 42% ± 15%, 38% ± 12%, 60% ± 17%, 24% ± 11%, and 22% ± 9.3%, respectively. However, heart rate was significantly higher (116% ± 9.2%, P = 0.003) in the upright position compared with the supine position.

DATA CONCLUSION

The effect of gravity during postural change from a supine to an upright position significantly decreases the PVF. Multiposture MRI allows acquisition of more detailed information on liver function.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;50:83-87.

Impaired lymphocyte function in patients with hepatic malignancies after selective internal radiotherapy

The purpose of our study was to assess the immune function of patients with inoperable hepatic malignancies after treatment with selective internal radiotherapy (SIRT) and to identify possible correlations with clinical parameters. In 25 patients receiving SIRT lymphocyte proliferation and the production of pro- and anti-inflammatory cytokines (interferon-γ and interleukin-10) after stimulation with mitogens and microbial antigens were tested prior to therapy, directly after therapy (day 1) and at day 2, 7 and 28 post therapy using the lymphocyte transformation test and enzyme-linked immunospot assays. Absolute counts and percentages of leukocyte and lymphocyte subsets were determined by flow cytometry. The most prominent finding was an immediate and significant (p < 0.05) decrease of lymphocyte proliferation and interferon-γ production directly after therapy which lasted until day 28 and was stronger upon stimulation with microbial antigens than with mitogens. Moreover, lymphopenia was revealed, affecting all lymphocyte subsets (CD3+, CD4+, CD8+ T cells, CD4+ CD8+ T cells, B cells and NK cells). SIRT led to a reduction in the percentage of activated HLA-DR+ monocytes and of CD45R0+ memory T cells. Higher radiation activity, the presence of liver cirrhosis, chronic kidney disease, diabetes mellitus and metastases were unfavorable factors for immunocompetence, while a better Eastern Cooperative Oncology Group performance status was associated with stronger immunological reactions. In conclusion, SIRT leads to severe impairment of cellular in vitro immune responses. Further studies are needed to assess a potential clinical impact.

Cardiac-gated, phase contrast magnetic resonance angiography is a reliable and reproducible technique for quantifying blood flow in canine major cranial abdominal vessels

Blood flow changes in cranial abdominal vessels are important contributing factors for canine hepatic disease. This prospective, experimental, pilot study aimed to evaluate cardiac-gated, phase contrast magnetic resonance angiography (PCMRA) as a method for characterizing blood flow in canine major cranial abdominal vessels. Eleven, healthy, adult beagle dogs were sampled. Cardiac-gated, phase contrast magnetic resonance angiography of the cranial abdomen was performed in each dog and blood flow was independently measured in each of the major cranial abdominal vessels by three observers, with two observers recording blood flow values once and one observer recording blood flow values three times. Each dog then underwent ultrasonographic examination of the liver with fine needle aspirations and biopsies submitted to cytologic and histologic examination. The mean absolute stroke volume and velocity were respectively 9.6 ± 1.9 ml and -11.1 ± 1.1 cm/s for the cranial abdominal aorta, 2.1 ± 0.6 ml and -6.6 ± 1.9 cm/s for the celiac artery, and 2.3 ± 1.0 ml and -7.9 ± 3.1 cm/s for the cranial mesenteric artery. The mean absolute stroke volume and velocity were respectively 6.7 ± 1.3 ml and 3.9 ± 0.9 cm/s for the caudal vena cava and 2.6 ± 0.9 ml and 3.2 ± 1.2 cm/s for the portal vein. Intraobserver reliability was excellent (intraclass correlation coefficient > 0.9). Interobserver reproducibility was also excellent (intraclass correlation coefficient 0.89-0.99). Results of liver ultrasonography, cytology, and histopathology were unremarkable. Findings indicated that cardiac-gated, phase contrast magnetic resonance angiography is a feasible technique for quantifying blood blow in canine major cranial abdominal vessels. Blood flow values from this sample of healthy beagles can be used as background for future studies on canine hepatic disease.

Cardiac 4D phase-contrast CMR at 9.4 T using self-gated ultra-short echo time (UTE) imaging

BACKGROUND

Time resolved 4D phase contrast (PC) cardiovascular magnetic resonance (CMR) in mice is challenging due to long scan times, small animal ECG-gating and the rapid blood flow and cardiac motion of small rodents. To overcome several of these technical challenges we implemented a retrospectively self-gated 4D PC radial ultra-short echo-time (UTE) acquisition scheme and assessed its performance in healthy mice by comparing the results with those obtained with an ECG-triggered 4D PC fast low angle shot (FLASH) sequence.

METHODS

Cardiac 4D PC CMR images were acquired at 9.4 T in healthy mice using the proposed self-gated radial center-out UTE acquisition scheme (TE/TR of 0.5 ms/3.1 ms) and a standard Cartesian 4D PC imaging sequence (TE/TR of 2.1 ms/5.0 ms) with a four-point Hadamard flow encoding scheme. To validate the proposed UTE flow imaging technique, experiments on a flow phantom with variable pump rates were performed.

RESULTS

The anatomical images and flow velocity maps of the proposed 4D PC UTE technique showed reduced artifacts and an improved SNR (left ventricular cavity (LV): 8.9 ± 2.5, myocardium (MC): 15.7 ± 1.9) compared to those obtained using a typical Cartesian FLASH sequence (LV: 5.6 ± 1.2, MC: 10.1 ± 1.4) that was used as a reference. With both sequences comparable flow velocities were obtained in the flow phantom as well as in the ascending aorta (UTE: 132.8 ± 18.3 cm/s, FLASH: 134.7 ± 13.4 cm/s) and pulmonary artery (UTE: 78.5 ± 15.4 cm/s, FLASH: 86.6 ± 6.2 cm/s) of the animals. Self-gated navigator signals derived from information of the oversampled k-space center were successfully extracted for all animals with a higher gating efficiency of time spent on acquiring gated data versus total measurement time (UTE: 61.8 ± 11.5%, FLASH: 48.5 ± 4.9%).

CONCLUSIONS

The proposed self-gated 4D PC UTE sequence enables robust and accurate flow velocity mapping of the mouse heart in vivo at high magnetic fields. At the same time SNR, gating efficiency, flow artifacts and image quality all improved compared to the images obtained using the well-established, ECG-triggered, 4D PC FLASH sequence.

Geometric modeling of hepatic arteries in 3D ultrasound with unsupervised MRA fusion during liver interventions

PURPOSE

Modulating the chemotherapy injection rate with regard to blood flow velocities in the tumor-feeding arteries during intra-arterial therapies may help improve liver tumor targeting while decreasing systemic exposure. These velocities can be obtained noninvasively using Doppler ultrasound (US). However, small vessels situated in the liver are difficult to identify and follow in US. We propose a multimodal fusion approach that non-rigidly registers a 3D geometric mesh model of the hepatic arteries obtained from preoperative MR angiography (MRA) acquisitions with intra-operative 3D US imaging.

METHODS

The proposed fusion tool integrates 3 imaging modalities: an arterial MRA, a portal phase MRA and an intra-operative 3D US. Preoperatively, the arterial phase MRA is used to generate a 3D model of the hepatic arteries, which is then non-rigidly co-registered with the portal phase MRA. Once the intra-operative 3D US is acquired, we register it with the portal MRA using a vessel-based rigid initialization followed by a non-rigid registration using an image-based metric based on linear correlation of linear combination. Using the combined non-rigid transformation matrices, the 3D mesh model is fused with the 3D US.

RESULTS

3D US and multi-phase MRA images acquired from 10 porcine models were used to test the performance of the proposed fusion tool. Unimodal registration of the MRA phases yielded a target registration error (TRE) of [Formula: see text] mm. Initial rigid alignment of the portal MRA and 3D US yielded a mean TRE of [Formula: see text] mm, which was significantly reduced to [Formula: see text] mm ([Formula: see text]) after affine image-based registration. The following deformable registration step allowed for further decrease of the mean TRE to [Formula: see text] mm.

CONCLUSION

The proposed tool could facilitate visualization and localization of these vessels when using 3D US intra-operatively for either intravascular or percutaneous interventions to avoid vessel perforation.

Vascular assessment of liver disease-towards a new frontier in MRI

Complex haemodynamic phenomena underpin the pathophysiology of chronic liver disease. Non-invasive MRI-based assessment of hepatic vascular parameters therefore has the potential to yield meaningful biomarkers for chronic liver disease. In this review, we provide an overview of vascular sequelae of chronic liver disease amenable to imaging evaluation and describe the current supportive evidence, strengths and the limitations of MRI methodologies, including dynamic contrast-enhanced, dynamic hepatocyte-specific contrast-enhanced, phase-contrast, arterial spin labelling and MR elastography in the assessment of hepatic vascular parameters. We review the broader challenges of quantitative hepatic vascular MRI, including the difficulties of motion artefact, complex post-processing, long acquisition times, validation and limitations of pharmacokinetic models, alongside the potential solutions that will shape the future of MRI and deliver this new frontier to the patient bedside.

Use of Caval Subtraction 2D Phase-Contrast MR Imaging to Measure Total Liver and Hepatic Arterial Blood Flow: Preclinical Validation and Initial Clinical Translation

Caval subtraction phase-contrast MR imaging is technically feasible and may offer a reproducible and clinically viable method for measuring total liver blood flow and hepatic arterial flow.

Purpose

To validate caval subtraction two-dimensional (2D) phase-contrast magnetic resonance (MR) imaging measurements of total liver blood flow (TLBF) and hepatic arterial fraction in an animal model and evaluate consistency and reproducibility in humans.

Materials and Methods

Approval from the institutional ethical committee for animal care and research ethics was obtained. Fifteen Sprague-Dawley rats underwent 2D phase-contrast MR imaging of the portal vein (PV) and infrahepatic and suprahepatic inferior vena cava (IVC). TLBF and hepatic arterial flow were estimated by subtracting infrahepatic from suprahepatic IVC flow and PV flow from estimated TLBF, respectively. Direct PV transit-time ultrasonography (US) and fluorescent microsphere measurements of hepatic arterial fraction were the standards of reference. Thereafter, consistency of caval subtraction phase-contrast MR imaging–derived TLBF and hepatic arterial flow was assessed in 13 volunteers (mean age, 28.3 years ± 1.4) against directly measured phase-contrast MR imaging PV and proper hepatic arterial inflow; reproducibility was measured after 7 days. Bland-Altman analysis of agreement and coefficient of variation comparisons were undertaken.

Results

There was good agreement between PV flow measured with phase-contrast MR imaging and that measured with transit-time US (mean difference, −3.5 mL/min/100 g; 95% limits of agreement [LOA], ±61.3 mL/min/100 g). Hepatic arterial fraction obtained with caval subtraction agreed well with those with fluorescent microspheres (mean difference, 4.2%; 95% LOA, ±20.5%). Good consistency was demonstrated between TLBF in humans measured with caval subtraction and direct inflow phase-contrast MR imaging (mean difference, −1.3 mL/min/100 g; 95% LOA, ±23.1 mL/min/100 g). TLBF reproducibility at 7 days was similar between the two methods (95% LOA, ±31.6 mL/min/100 g vs ±29.6 mL/min/100 g).

Conclusion

Caval subtraction phase-contrast MR imaging is a simple and clinically viable method for measuring TLBF and hepatic arterial flow.

Online supplemental material is available for this article.

Intrahepatic portal vein blood volume estimated by non-contrast magnetic resonance imaging for the assessment of portal hypertension

PURPOSE

To investigate the feasibility of estimating the portal vein blood volume that flows into the intrahepatic volume (IHPVBV) in each cardiac cycle using non-contrast MR venography technique as a surrogate marker of portal hypertension (PH).

MATERIALS AND METHODS

Ten patients with chronic liver disease and clinical symptoms of PH (40% males, median age: 54.0, range: 44-73 years old) and ten healthy volunteers (80% males, median age: 54.0, range: 44-66 years old) were included in this study. A non-contrast Triple-Inversion-Recovery Arterial-Spin-Labeling (TIR-ASL) technique was used to quantify the IHPVBV in one and two cardiac cycles. Liver (LV) and spleen volumes (SV) were measured by manual segmentation from anatomical MR images as morphological markers of PH. All images were acquired in a 1.5T Philips Achieva MR scanner.

RESULTS

PH patients had larger SV (P=0.02) and lower liver-to-spleen ratio (P=0.02) compared with healthy volunteers. The median IHPVBV in healthy volunteers was 13.5cm(3) and 26.5cm(3) for one and two cardiac cycles respectively, whereas in PH patients a median volume of 3.1cm(3) and 9.0cm(3) was observed. When correcting by LV, the IHPVBV was significantly higher in healthy volunteers than PH patients for one and two cardiac cycles. The combination of morphological information (liver-to-spleen ratio) and functional information (IHPVBV/LV) can accurately identify the PH patients with a sensitivity of 90% and specificity of 100%.

CONCLUSION

Results show that the portal vein blood volume that flows into the intrahepatic volume in one and two cardiac cycles is significantly lower in PH patients than in healthy volunteers and can be quantified with non-contrast MRI techniques.

Therapeutic novelties of inhaled corticosteroids and bronchodilators in asthma

Orally inhaled agents are a key therapeutic class for treatment of asthma. Inhaled corticosteroids (ICS) are the most effective anti-inflammatory treatment for asthma thus representing the first-line therapy and bronchodilators complement the effects of ICSs. A significant body of evidence indicates that addition of a β2-agonist to ICS therapy is more effective than increasing the dose of ICS monotherapy. In this paper, pharmacological features of available ICSs and bronchodilators will be reviewed with a focus on fluticasone propionate/formoterol fumarate combination which represents the one of the most powerful ICS acting together with the most rapid active LABA.

Quantification of hepatic blood flow using a high-resolution phase-contrast MRI sequence with compressed sensing acceleration

OBJECTIVE. The objective of our study was to evaluate the performance of a high-spatial-resolution 2D phase-contrast (PC) MRI technique accelerated with compressed sensing for portal vein (PV) and hepatic artery (HA) flow quantification in comparison with a standard PC MRI sequence. SUBJECTS AND METHODS. In this prospective study, two PC MRI sequences were compared, one with parallel imaging acceleration and low spatial resolution (generalized autocalibrating partial parallel acquisition [GRAPPA]) and one with compressed sensing acceleration and high spatial resolution (sparse). Seventy-six patients were assessed, including 37 patients with cirrhosis. Two observers evaluated PC image quality. Quantitative analyses yielded a mean velocity, flow, and vessel area for the PV and HA and an arterial fraction. The PC techniques were compared using the paired Wilcoxon test and Bland-Altman statistics. The sensitivity of the flow parameters to the severity of cirrhosis was also assessed. RESULTS. Vessel delineation was significantly improved using the PC sparse sequence (p < 0.034). For both in vitro and in vivo measurements, PC sparse yielded lower estimates for vessel area and flow, and larger differences between PC GRAPPA and PC sparse were observed in the HA. PV velocity and flow were significantly lower in patients with cirrhosis on both PC sparse (p < 0.001 and p = 0.042, respectively) and PC GRAPPA (p < 0.001 and p = 0.005, respectively). PV velocity correlated negatively with Child-Pugh class (r = -0.50, p < 0.001), whereas the arterial fraction measured with PC sparse was higher in patients with Child-Pugh class B or C disease than in those with Child-Pugh class A disease, with a trend toward significance (p = 0.055). CONCLUSION. A high-spatial-resolution highly accelerated compressed sensing technique (PC sparse) allows total hepatic blood flow measurements obtained in 1 breath-hold, provides improved delineation of the hepatic vessels compared with a standard PC MRI sequence (GRAPPA), and can potentially be used for the noninvasive assessment of liver cirrhosis.

Temporal assessment of pancreatic blood flow and perfusion following secretin stimulation using noninvasive MRI

PURPOSE

To dynamically quantify pancreatic perfusion and flow within the arteries supplying the pancreas in response to secretin stimulation.

MATERIALS AND METHODS

Twelve healthy male subjects were scanned at 1.5T with arterial spin labeling to measure tissue perfusion and phase contrast magnetic resonance imaging (MRI) to measure vessel flow. Superior mesenteric (SMA), gastroduodenal (GDA), common hepatic (HA), and splenic (SA) arterial flow and pancreatic perfusion were serially measured for 50 minutes following 1 IU/kg intravenous secretin. The significance of differences between timepoints was tested using a repeated measures one-way analysis of variance (ANOVA).

RESULTS

Baseline blood flow (mean ± SEM or median [IQR]) for SMA, HA, SA, and GDA was 7.6 ± 1.3, 4.0 ± 0.5, 8.2 ± 0.8, and 0.9 (0.8-1.4) ml/s, respectively. Baseline pancreatic perfusion was 200 ± 25 ml/100g/min. Blood flow increased in the SMA (234%, P < 0.0001) and GDA (155%, P = 0.015) immediately after secretin injection. Reduced HA blood flow was observed after 10 minutes (P = 0.066) with no change in SA flow (P = 0.533). Increased pancreatic perfusion was maintained for 40 minutes after injection with a maximal increase at 5 minutes (16.8%, P = 0.025).

CONCLUSION

Intravenous secretin resulted in significant temporal changes in pancreatic perfusion and arterial blood flow.

Velocity mapping of the aortic flow at 9.4 T in healthy mice and mice with induced heart failure using time-resolved three-dimensional phase-contrast MRI (4D PC MRI)

Objectives

In this study, we established and validated a time-resolved three-dimensional phase-contrast magnetic resonance imaging method (4D PC MRI) on a 9.4 T small-animal MRI system. Herein we present the feasibility of 4D PC MRI in terms of qualitative and quantitative flow pattern analysis in mice with transverse aortic constriction (TAC).

Materials and methods

4D PC FLASH images of a flow phantom and mouse heart were acquired at 9.4 T using a four-point phase-encoding scheme. The method was compared with slice-selective PC FLASH and ultrasound using Bland–Altman analysis. Advanced 3D streamlines were visualized utilizing Voreen volume-rendering software.

Results

In vitro, 4D PC MRI flow profiles showed the transition between laminar and turbulent flow with increasing velocities. In vivo, 4D PC MRI data of the ascending aorta and the pulmonary artery were confirmed by ultrasound, resulting in linear regressions of R² > 0.93. Magnitude- and direction-encoded streamlines differed substantially pre- and post-TAC surgery.

Conclusions

4D PC MRI is a feasible tool for in vivo velocity measurements on high-field small-animal scanners. Similar to clinical measurement, this method provides a complete spatially and temporally resolved dataset of the murine cardiovascular blood flow and allows for three-dimensional flow pattern analysis.

Electronic supplementary material

The online version of this article (doi:10.1007/s10334-014-0466-z) contains supplementary material, which is available to authorized users.

Magnetic resonance imaging of the liver: apparent diffusion coefficients from multiexponential analysis of b values greater than 50 s/mm2 do not respond to caloric intake despite increased portal-venous blood flow

PURPOSE

The purpose of this study was to measure potential changes of the apparent diffusion coefficient (ADC) in diffusion-weighted imaging of the liver before and after caloric challenge in correlation to the induced changes in portal vein flow.

MATERIALS AND METHODS

The study was approved by the local ethics committee. Each of 10 healthy volunteers underwent 4 measurements in a 1.5-T whole-body magnetic resonance scanner on 2 different days: a first scan after fasting for at least 8 hours and a second scan 30 minutes after intake of a standardized caloric either a protein- or carbohydrate-rich meal. Diffusion-weighted spin-echo echo-planar magnetic resonance images were acquired at b values of 0, 50, 150, 250, 500, 750, and 1000 s/mm. In addition, portal vein flow was quantified with 2-dimensional phase-contrast imaging (velocity encoding parallel to flow direction, 60 cm/s). Mean ADC values for regions of interest in 3 different slices were measured from b50 to b250 and from b500 to b1000 images.

RESULTS

Carbohydrate- and protein-rich food intake both resulted in a substantial increase in the portal vein flow (fasting state, 638.6 ± 202.3 mL/min; after protein intake, 1322 ± 266.8; after carbohydrate intake, 1767 ± 421.6). The signal decay with increasingly strong diffusion weighting (b values from 0 to 1000 s/mm2) exhibited a triexponential characteristic, implying fast, intermediate, and slow-moving water-molecule proton-spin ensembles in the liver parenchyma. Mean ADC for high b values (b500-b1000) after fasting was 0.93 ± 0.09 × 10 mm/s; that after protein intake, 0.93 ± 0.11 × 10; and that after carbohydrate intake, 0.93 ± 0.08 × 10. For intermediate b values (b50-b250), the signal-decay constants were 1.27 ± 0.14 × 10 mm/s, 1.28 ± 0.15 × 10, and 1.31 ± 0.09 × 10, respectively. There was no statistically significant difference between fasting and caloric challenge.

CONCLUSIONS

The postprandial increase in portal vein flow is not accompanied by a change of liver parenchymal ADC values. In clinical diffusion imaging, patients may be scanned without prescan food-intake preparations. To minimize interference of perfusion effects, liver-tissue molecular water diffusion should be quantified using high b values (≥500 s/mm) only.

Clinical translation in the virtual liver network

The liver is the central detoxifying organ, continuously removing xenobiotics from the vascular system. Given its role in drug metabolism, a functional understanding of liver physiology is crucial to optimizing drug efficacy and patient safety. The Virtual Liver Network (VLN), a German national flagship research program, focuses on producing validated computer models of human liver physiology. These models are used to analyze patient-derived data and thereby gain mechanistic insights in the processes underlying drug pharmacokinetics (PK).

Laser speckle contrast imaging for intraoperative assessment of liver microcirculation: a clinical pilot study

Background

Liver microcirculation can be affected by a wide variety of causes relevant to liver transplantation and resectional surgery. Intraoperative assessment of the microcirculation could possibly predict postoperative outcome. The present pilot study introduces laser speckle contrast imaging (LSCI) as a new clinical method for assessing liver microcirculation.

Methods

LSCI measurements of liver microcirculation were performed on ten patients undergoing liver resection. Measurements were made during apnea with and without liver blood inflow occlusion. Hepatic blood flow was assessed by subtracting zero inflow signal from the total signal. Zero inflow signal was obtained after hepatic artery and portal vein occlusion. Perfusion was expressed in laser speckle perfusion units, and intraindividual and interindividual variability in liver perfusion was investigated using the coefficient of variability.

Results

Hepatic microcirculation measurements were successfully made in all patients resulting in analyzable speckle contrast images. Mean hepatic blood flow was 410±36 laser speckle perfusion units. Zero inflow signal amounted to 40%±4% of the total signal. Intraindividual and interindividual coefficients of variability in liver perfusion were 25% and 28%, respectively.

Conclusion

Under the conditions of this pilot study, LSCI allows rapid noncontact measurements of hepatic blood perfusion over wide areas. More studies are needed on methods of handling movement artifacts.

Inhaled corticosteroid and long-acting β2-agonist pharmacological profiles: effective asthma therapy in practice

Fixed-dose combinations of inhaled corticosteroids (ICSs) and long-acting β2-agonists (LABAs) have been used to manage asthma for several years. They are the preferred therapy option for patients who do not achieve optimal control of their asthma with low-dose ICS monotherapy. In Europe, four ICS/LABA products are commercially available for asthma maintenance therapy (fluticasone propionate/formoterol fumarate, fluticasone propionate/salmeterol xinafoate, budesonide/formoterol fumarate and beclometasone dipropionate/formoterol fumarate), and other combinations are likely to be developed over the next few years (e.g. mometasone/formoterol fumarate, fluticasone furoate/vilanterol, mometasone/indacaterol). Data from randomized, controlled, clinical trials do not demonstrate a clear overall efficacy difference among ICS/LABA combinations approved for asthma therapy. Conversely, pharmacological data indicate that there may be certain advantages to using one ICS or LABA over another because of the specific pharmacodynamic and pharmacokinetic profiles associated with particular treatments. This review article summarizes the pharmacological characteristics oft he various ICSs and LABAs available for the treatment of asthma, including the potential for ICS and LABA synergy, and gives an insight into the rationale for the development of the latest ICS/LABA combination approved for asthma maintenance therapy.

MR imaging evaluation of the hepatic vasculature

Assessment of the hepatic vasculature is essential for tumor staging, surgical planning, and understanding of liver disease. Technological advances have made contrast-enhanced magnetic resonance (MR) imaging comparable to multidetector-row computed tomography for diagnostic vascular imaging with respect to spatial resolution. Unenhanced MR angiographic sequences enable reasonable clinical assessment of vessels without contrast agents in patients with contraindications or renal insufficiency. Furthermore, MR angiography may be used to provide directional information through manipulation of the signal intensity of flowing blood. A major limitation to consistent contrast-enhanced MR angiography is the timing of MR image acquisition with arrival of the contrast bolus in the structures of interest. In this article, the authors discuss currently available techniques for imaging of the hepatic vasculature.

An object-oriented modelling framework for the arterial wall

An object-oriented modelling framework for the arterial wall is presented. The novelty of the framework is the possibility to generate customizable artery models, taking advantage of imaging technology. In our knowledge, this is the first object-oriented modelling framework for the arterial wall. Existing models do not allow close structural mapping with arterial microstructure as in the object-oriented framework. In the implemented model, passive behaviour of the arterial wall was considered and the tunica adventitia was the objective system. As verification, a model of an arterial segment was generated. In order to simulate its deformation, a matrix structural mechanics simulator was implemented. Two simulations were conducted, one for an axial loading test and other for a pressure-volume test. Each simulation began with a sensitivity analysis in order to determinate the best parameter combination and to compare the results with analogue controls. In both cases, the simulated results closely reproduced qualitatively and quantitatively the analogue control plots.